Method and apparatus for creating a free-form three-dimensional article using a layer-by-layer deposition of molten metal and using a stress-reducing annealing process on the deposited metal

ABSTRACT

A method and apparatus for the accurate formation of a three-dimensional article comprises providing a supply of substantially uniform size droplets of a desired material wherein each droplet has a positive or negative charge. The supply of droplets is focused or aligned into a narrow stream by passing the droplets through or adjacent an alignment means which repels each droplet toward an axis extending through the alignment means. The droplets are deposited in a predetermined pattern at a predetermined rate onto a target to form the three-dimensional article without the use of a mold of the shape of the three-dimensional article. A means for reducing stress anneals portions of the deposited droplets which form a newly formed surface of the three-dimensional article.

FIELD OF THE INVENTION

The present invention relates generally to a method and apparatus fordepositing molten metal in a layer-by-layer process on a target such asa work space platform or a substrate to form a three-dimensional object.

Co-pending applications directed to other embodiments of this inventionare disclosed in co-pending applications which are owned by the sameAssignee herein, Aeroquip Corporation, Ser. Nos. 08/525,925; 08/526,071;08/526,070 and 08/525,000 now U.S. Pat. No. 5,617,911 issued Apr. 8,1997.

BACKGROUND OF THE INVENTION

Various methods have been proposed to form three-dimensional articles bydepositing of layers of material on a substrate. This layeringmanufacturing is also known as solid free-form fabrication or rapidprototyping. A computer model of a desired object is sliced into afinite set of layers. The layers are created sequentially, or bondedonto a previously formed layer. This layering creates an object whichapproximates the intended geometry of the three-dimensional object, in amuch as the layers cause a "staircase effect" at the edge or peripheralarea of the object. The staircase effect is an artifact of the processof applying the discrete layers of material. The final appearance of theobject can be improved by minimizing the layer thickness or by usingsuch additional processing steps as sand blasting and the like whichsmooth out the surface of the object.

Stereolithography is one method for forming a three-dimensionalpolymeric article made of a polymeric material. In stereolithography, aphotopolymer is selectively cured using a laser beam to create eachlayer. The three-dimensional article is built up on an elevator-typeplatform in a vat containing the liquid photopolymer. Successive layersare created by lowering the partially created objects into thephotopolymer liquid and laser curing a new layer of photopolymermaterial on the top of the partially built object.

Another method comprises a fused-deposition modeling process which meltsand extrudes a polymer substance through a nozzle onto a target to formthe three-dimensional object.

Still another method involves lamination wherein layers of a paper orpolymer material are cut and bonded to the substrate, then trimmed onthe edges or peripheries with a laser to correspond to a desired layeror cross-section through the article. The unwanted or waste areas ofeach layer are cut into a grid. These "squares" stack up to form squareprisms, truncated by the boundary of the object. The "square" areas arephysically removed after product completion, leaving only the desiredpart.

Still another method uses a lamination process wherein papers or othermask-type materials are used to build layers of material. A laser beamcuts the layer geometry into the paper which is used as a mask. Thematerial is deposited in the cutout area which defines a single layer ofthe three-dimensional article and portions of the material overlaps ontothe mask material.

In the past, it has been difficult to form three-dimensional articlesmade of a metal by using a free-form fabrication or deposition layeringprocess.

One metal fabrication method involves a laser sintering process whichspreads a layer of a metal powder material on top of partial objects andthen selectively sinters (using a laser beam) the portion whichcomprises the new layer.

Another metal fabrication method involves a post sintering processwherein metal powder materials are bonded together with a polymerbinding material. However, it is difficult to entirely remove thepolymer binding material from the finished three-dimensional object. Thepresence of binder residue detracts from the desired strength and otherproperties of the metal object. In addition, removal of the polymermaterial causes voids in the three-dimensional object such that theobject is somewhat porous. Another metal material (such as a lowertemperature metal) can be infiltrated into the porous three-dimensionalobject with temperature differentiation, in an attempt to fill thevoids. However, the three-dimensional object then has a "honey-comb"type composite structure of less than desirable properties and issubject to creeping or warping during sintering of the original hostmaterial. In addition, the presence of residual polymer and/or the fillmaterial act as contaminates within the three-dimensional object andthereby affect the properties of the object. The contaminates mayinclude products of oxidation, excess carbon, binder residue and thelike. It is to be understood that the use of filling or fiber elementsin the infiltration process is different from the use of alloymaterials. In infiltration the two materials remain distinct; whereas,in an alloy the materials are homogeneously blended together to achievea desirable combination of properties. Another concern is that wheneverthe infiltrated material bonds imperfectly with the matrix material, themicrostructure has a very large number of stress concentrators, therebyreducing the strength of the object. While such three-dimensional"infiltrated" objects are sometimes called "fully dense" objects, such aterm is misdescriptive of the actual characteristics of thethree-dimensional object since the three-dimensional object is notcomprised of substantially one type of a preferred metal.

Still other fabrication methods use metal deposition techniques inconjunction with a metal removal technique such as milling, grinding,sand blasting and the like. The "staircase" effect and the roughness atthe edge of each layer are eliminated by machining each layer and itsperipheries after the layer is deposited. It is the machining or metalremoval process that determines the actual dimensional accuracy of thethree-dimensional object.

Currently there are several methods and apparatuses for depositingmolten material. For example, the Mertz et al. U.S. Pat. No. 5,281,789describes a welding process and an apparatus for depositing moltenmetal. A molten metal is deposited on a work surface and subsequentlayers of metal are deposited thereon. An electrode and weld torch arepreferably movable as a unit so that the molten metal may be depositedonto selective locations on the work surface. Alternatively, the worksurface may be moveable while the weld torch and the collector electrodeare moveable or held stationary so as to selectively position thedeposited material on the work surface. The droplet size is controlledby applying additional mechanical energy to the feed wire to constantlyvibrate the feed metal.

The Prinz et al. U.S. Pat. No. 5,286,573 describes a method usingsupport structures for the creation of objects by a layer depositionprocess. In the deposition process, each layer is composed of twoportions. One portion represents a cross-sectional slice of athree-dimensional object being built ("the object") and is composed ofthe desired deposition material or materials. The other portion is thecomplement of the object shape of the first portion and serves as asupport structure which supports the growing object form ("thesupport"). The object material and the support structure material areeach applied in a predetermined sequence. A plurality of layers, eachplaced upon the previous layer, is formed. In this way, a layeredstructure is built up. The layered structure contains the object made ofthe deposition material surrounded by the support material. For eachlayer, both, or one of, or neither of the support material and theobject material can be shaped to produce its desired object. Preferably,the shaping occurs after the object or support material is applied andbefore the subsequent layer is applied.

The Prinz et al. U.S. Pat. No. 5,301,863 describes an automated systemhaving multiple work stations for forming objects by incremental buildupof layers. Each layer a cross-sectional slice of a three-dimensionalobject being built and is composed of the desired object material. Inaddition to the object material, each layer usually also contains asecond portion that acts as a complement of the object shape of thedeposition material portion and serves as a support structure for thegrowing object form. During the manufacture of the article, severaloperations are performed on the workpiece for each layer. In addition tothe material deposition station, a plurality of processing stations areemployed, each of which has at least one separate function. Thesefunctions can include any combination of shot peening, cleaning,blasting, heat treating, shaping, inspection, mask making and packaging.

The Prinz et al. U.S. Pat. No. 5,301,415 describes a method for thefabrication of three-dimensional articles by incremental materialbuild-up of layers of material. In one embodiment, a layer of object andsupport material is applied. Depending on the shape of the object,either one or the other material is applied first, then shaped toachieve dimensional accuracy, and then the other material is deposited.The deposited layer is then machined, cleaned, shot-peened and the like.The process is repeated until all layers have been placed. After thefinal layer has been applied, the complementary material is removedleaving the created object formed of the deposition material.

The deAngelis U.S. Pat. No. 5,398,193 describes a method and anapparatus for making a three-dimensional object through controlledlayer-by-layer deposition and/or extraction. A three-dimensionalcomputer model representation of the three-dimensional part is providedand the model representation is sliced into a plurality of successivelayers corresponding to layers of predetermined thicknesses of the part.The computer model generates sequences of the part and any complementarysupport material contours which correspond to each layer. Materials forone or more contours are deposited onto a work surface within aprocessing enclosure. Portions of the material are removed from thecontours. The deposition processing and removing steps are repeated asnecessary under the control of the computer model to complete thethree-dimensional object. Further processing includes machining off asublayer to ensure thickness tolerances or roughening and chemicallyenhancing the surface to ensure selective binding to the next aggregatelayer. The controlled layer creation steps are repeated to build theentire part surrounded by complementary materials which are then removedto obtain a fabricated part.

A major disadvantage of the above methods is that the machining portionof these methods is relied upon to achieve the desired dimensionalaccuracy of the three-dimensional object. In many situations, theobjects being formed require multiple post fabrication steps to producean acceptable three-dimensional object or end product.

There is a need for an improved method for creating three-dimensional orsolid objects which utilizes accurate deposition of the material onto awork surface or substrate. However, until the present invention, therehas been no disclosure or suggestion that a supply of droplets could beaccurately controlled and dispensed to form a high qualitythree-dimensional or solid article in a net shape without the use of acollector or mold.

One method for forming a spray of substantially uniform size droplets isdisclosed in the Chun et al. U.S. Pat. No. 5,266,098 which describes aprocess and an apparatus for producing and maintaining charged,uniformly sized metal droplets. The droplets are deposited as a spray tocoat a substrate. A droplet generator is disposed within a spraychamber. The droplet generator comprises a container for holding andliquefying a charge of metal, a means for forming uniformly sized metaldroplets, and a means for charging the metal droplets as the dropletsare formed. The forming means is preferably either a vibrating means forvibrating the molten metal in the container (or at least one oscillatinggas jet disposed outside the container at the point where the liquifiedmetal exits the container). The liquified metal is forced from thecrucible through an orifice in the container so as to form the metaldroplets. As the liquified metal exits at least one orifice as a jet orstream, the imposed vibrations in the liquified metal cause the jet tobreak up into uniformly sized metal droplets. An electrical charge isapplied to the droplets as the droplets are being formed. The metaldroplets may be charged by either charging the liquified metal while inthe container or by charging the droplets as, or after, the droplets areformed after exiting the crucible. As each droplet breaks from the jetor stream, the droplet retains a portion of the charge. With thatcharge, the droplets repel each other in flight and scatter into acone-shape as the droplets fall toward a substrate. When the uniformlysized droplets are charged, the droplets are oriented to form a coneconfiguration due to the like polarity of the droplets and the repellingof each droplet from its neighboring droplet. The Chun et al. '098patent further claims the application of an electric field in the flowpath of the metal droplets to change their trajectories.

A thesis submitted by C. H. Passow to the Department of MechanicalEngineering at the Massachusetts Institute of Technology (MIT) on May 5,1992 describes a study of spray forming using uniform droplets sprays,droplet placement production techniques, and droplets selection anddeflection wherein parallel plates are positioned below the chargingplate to deflect the charged droplets off to the side where they wouldbe collected. Uncharged droplets would pass unhindered.

An article by P. J. Acquaviva et al. entitled Issues in Application ofThermo Spraying to Melt Mold Fabrication published in IBECInternational, 1994, describes a uniform droplet spray and depositionprocess which can be manipulated by moving a substrate at various speedsand directions.

A thesis submitted by Godard Karl Abel to the Department of MechanicalEngineering at MIT on May 18, 1994 describes using a uniform dropletspray forming process to form deposits on stationary and movingsubstrates; the spray forming of three-dimensional parts; and, insteadof allowing droplets to scatter randomly based on an unknowndisturbance, the droplets could be charged to varying amounts and thendeflected to create a more predictable mass flux distribution.

The Orme et al. U.S. Pat. Nos. 5,171,360; 5,226,948; 5,259,593; and5,340,090 describe methods and apparatuses for forming a net formproduct by directing a stream of a liquid material onto a collector ofthe shape of the desired product. A time variable disturbance is appliedto the stream to produce a liquid droplet stream with the dropletsimpacting on the collector and solidifying into a unitary shape. TheOrme et al. 1995 paper presented at SFF in Austin, Tex. describesthermal design parameters for the development of solid free-formfabrication of structural materials with controlled droplets.

In view of the need for a better and more efficient method for themanufacturing and forming of three-dimensional solid objects, and as aresult of extensive research, a new method for creating athree-dimensional solid object by depositing a molten metal has now beendeveloped.

As far as is known, there is no disclosure that a three-dimensionalsolid object can be formed by dispensing uniformly sized metal dropletsincrementally in layers in a highly accurate manner.

Accordingly, it is an object of the present invention to develop anapparatus and process for manufacturing high quality solid metalobjects. The present invention further provides a process which does notinvolve the use of multiple processing steps to form each layer ofdeposition, or otherwise achieve dimensional accuracy of thethree-dimensional solid object.

BRIEF DESCRIPTION OF THE INVENTION

The present invention provides a highly accurate method and apparatusfor incrementally superimposing layers of uniformly sized metal dropletsof a desired material (or materials) onto a target or platform to form athree-dimensional solid object. The objects are created rapidly anddirectly by the controlled deposition of the droplets. The deposition ofthe droplets into layers is controlled using computer-based models ofthe object geometry. The solid object is built incrementally usinglayers created from uniformly-sized droplets. It is the control andapplication of the individual droplets which provides the solid objectwith its desired dimensional characteristics.

The method and apparatus of the present invention provide a"fully-dense" three-dimensional solid object which is comprised of ametal or desirable alloy material. In addition, the three-dimensionalsolid article has a uniform density and has substantially no voids andis not porous. No sintering or infiltrating processes are needed inorder to form the fully dense article of the present invention. Thesolid object formed using the present invention has a homogeneousmicrostructure. In addition, the article of the present invention issubstantially contaminate-free such that the article formed has highlydesirable physical characteristics. The article made according to thepresent invention possesses the desirable physical characteristics andproperties which are substantially better than those properties found inarticles formed by conventional casting, spraying or molding processes.

The three-dimensional solid object has desirable dimensional tolerances,tensile, fatigue and compressive strengths, ductility, toughness,hardness and wear-resistance characteristics and properties. The solidobject has a substantially uniform or homogeneous isotropic content ofthe material throughout the structure of the three-dimensional article.

The present invention can be utilized internally within a facility in avery rapid and economic fashion to produce metal tooling or products forthe customer's production requirements. In addition, the apparatus ofthe present invention can be built in "table top" size for use inapplications which have limited space availability such as submarines oroff-shore drilling rigs. Further, it is possible to quickly produce adesired three-dimensional solid object so that the customer need nothave costly space-consuming inventories always on hand.

In preferred embodiments, the objects are created using a uniformdroplet forming process such as described in the Chun et al. U.S. Pat.No. 5,226,098 wherein the deposition material is supplied into a metalfeed system which has a heater for melting the metal. The molten metalis contained in a droplet forming means such as a crucible having atleast one orifice which permits passage of the molten metaltherethrough. In a preferred embodiment, the orifice has a diameter in arange of about 50 to 500 microns. The molten material is subjected to apressure differential of about 5-50 psi which forces the molten materialthrough the orifice as a stream. The molten metal is subjected to acertain frequency and/or amplitude such that the liquified metalvibrates. The vibration and metal surface tension causes the controlledbreak up of the stream of molten metal into uniformly sized droplets asthe molten material is dispensed from the orifice. As the droplets areformed, the droplets are subjected to a positive or negative charge. Thelike charge on the individual droplets keep the droplets separate andprevents the droplets from merging together in flight with neighboringor adjacent droplets, and thus allows the droplets to maintain theiruniform size.

The present invention is an improvement over the Chun et al. '098technology wherein the like charge on the droplets causes the dropletsto spread out and to be deposited as a spray. The present inventionprovides a method for focusing the droplets or aligning the droplets ina narrow stream or single file after their formation rather thanallowing the droplets to be spread out into a spray. The presentinvention also prevents the droplets from merging vertically with eachother, thus further maintaining size consistency.

According to the present invention, the supply of droplets is aligned orfocused into a substantially narrow stream or line by passing the supplyof charged droplets adjacent or through an alignment means. Thealignment means adds an additional (electrical) force field by carryingthe same charge as the droplets. The alignment means repels the dropletssubstantially uniformly inward towards an axis extending through thealignment means. The repelling of the droplets inwardly forces thedroplets into a fine stream. By repelling the droplets throughout theflight of the droplets, the droplets remain focused in a single filestream or fine line of droplets.

Thus, it is to be further understood that the present inventioncomprises, in part, a method to align the droplets. In variousembodiments, the droplets can be aligned by either maintaining, reducingor increasing the charge on the droplets as the droplets are beingdispensed and deposited onto a target or partially formedthree-dimensional article.

In one preferred embodiment, the alignment means comprises at least oneaxisymmetric hollow repelling cylinder or frusto-conical repelling meanswhich is positioned adjacent or close to the target or partially formedarticle. An axis extending through the alignment means is aligned withthe nominal path of the stream of droplets.

The emerging stream of molten material has an electric charge of thesame polarity as each droplet. In a preferred embodiment, as the jetstream breaks into droplets, a charge is supplied to each droplet. Whenthe charging means is held at a predetermined voltage with respect tothe jet stream, the combination of the voltage and the capacitancebetween the charging means and the jet stream brings a charge to theleading point of the jet stream. Each droplet retains a charge that thedroplet held before it broke free from the jet stream. The charge on thedroplet causes each droplet to repel from adjacent droplets whichprevents the droplets from merging.

According to the present invention, the charged droplets are maintainedin a predetermined narrow line or path by the alignment means. Inpreferred embodiments, the alignment means maintains the charge, appliesa further charge to, or reduces the charge on the descending droplets.The like charge applied to the descending droplets keeps the dropletsaway from the alignment means and also away from each adjacent droplet.Thus, when the charge on the alignment means is sufficiently great, thedroplets will be a uniform distance from each other in the stream andwill tend to cluster around an axis extending through the alignmentmeans. The droplets are held in a substantially narrow stream as thedroplets pass through or adjacent the alignment means.

It is to be understood that in various embodiments, the alignment meanscan comprise an additional means for reducing the charge on thedescending droplets by supplying, for example, to a stream of positivelycharged droplets, an electron beam which reduces the charge on thedroplets.

In a preferred embodiment, the droplets are supplied onto the target inan enclosed system such that there is less risk of any contaminationsuch as oxidation occurring on the surface of the droplets and thereforewithin the layers of deposited materials. In one preferred embodiment,the air in the work space is replaced by an inert gas such as argon ornitrogen. When heavy inert gases such as argon are used, the inert gasesare preferably introduced at a lower end of the enclosed work space. Theheavy inert gases displace the lighter air which can flow out the top ofthe enclosed work space. In another preferred embodiment, the air in thework space is replaced by a lighter inert gas such as a nitrogen. It isto be understood however, that the type of inert gas depends upon thetype of metal being deposited. It is further understood that withdeposition of aluminum, it is preferable not to use nitrogen since thenitrogen and aluminum react. It is further understood that in variousembodiments, the density differences between the inert gas and theambient air can be accentuated by, for example, cooling the argon andheating the nitrogen. In a preferred embodiment, the work space is keptunder a positive pressure so that any leaks are outward and no ambientair leaks into the enclosed work space. Various containment means forenclosing the work space include a flexible shroud; for example made outof a suitable material such as polyvinylchloride and the like, which ismounted on a metal frame. Other means for enclosing the work spaceincludes a rigid transparent plastic housing made of a suitablematerial. These containment means may replace the large cumbersomevacuum chambers now in use such that the apparatus of the presentinvention can be used in a table top design. A vacuum chamber mayhowever be used to obtain desired droplet purity levels and resultingthree-dimensional part properties.

In a preferred embodiment, the work environment has an inert atmosphere.It is to be understood that a gas controlled system can be used to forcethe molten material from the crucible containing the molten material athigh pressures of about 20-50 psi (≈140-350 kPa). A further gascontrolled system provides a low pressure source at about 1-2 psi to thework environment. It is to be understood that these pressures are gaugepressures i.e., they are higher than atmospheric pressure, are notabsolute, and are merely shown for ease of illustration of the presentinvention. In operation, it is contemplated that if necessary, the workenvironment can be repeatedly purged by introducing an inert gas tolimit oxygen levels. It is to be understood that suitable minimum oxygenlevels are determined by measuring the properties of the end productsexposed to different contamination levels.

The fine stream or line of droplets is accurately positioned on a targetor work station to form the three-dimensional article. It is to beunderstood that according to the method of the present invention, eitherthe supply of the stream of droplets and/or the target can be moved toform the three-dimensional article. In one embodiment, the target can bemoved a predetermined distance in response to the line of droplets beingapplied to the target. For example, the target can be moved horizontallyand/or vertically at a predetermined rate which is dependent, at leastin part upon the droplet deposition rate. In certain preferredembodiments the droplet deposition rate can be monitored by avision/counting system. The three-dimensional article formed on the workstation is operatively connected to a positioning system. In a preferredembodiment, the positioning system comprises at least 2-3 axis, and incertain embodiments, a 3 to 5 axis table, drives for the axes, encodingmeans for receiving and transmitting positional data, and a controlsystem to coordinate motion along the axes. The control means cancomprise a computer-based representation of the object geometry whichsupplies the coordinates and means to interpret movement of the workstation. In an alternative embodiment, the droplet forming/supplyingmeans can also be operatively connected to a different positioning meanswhich moves the supply of droplets in about positions between at leastthe X, Y and Z axes in response to a predetermined pattern. In yetanother embodiment both the work station and the supply of droplets canbe moved to form the three-dimensional article.

In a preferred embodiment, a planning system, such as a suitablesoftware program accepts a solid-model representation of thethree-dimensional object. The planning system sections the objectgeometry into a finite number of slices and plots the deposition pathneeded to achieve each layer, including any overhang supportrequirements (as described in detail below) of the article as a whole.The control system coordinates the planning system, the movement of thedroplet forming/supply means, and the work station positioning system.The control system(s) also monitor(s) all sensor inputs which relayinformation on the various operating parameters and maintains correctoperating parameters such as pressures, temperatures, voltages and thelike.

According to the present invention, the parameters (such as thepressure, orifice diameter, frequency and amplitude of the vibrations ofthe droplets) can be varied to change the diameter of the uniformlysized droplets. It is to be understood that the optimum diameter of thedroplets depends, in part, upon the three-dimensional articles beingformed and upon the type of material being deposited. Other parameterssuch as the feed rate of the metal into the crucible, crucible pressure,temperature and amount of charge on the droplets also affect the sizeand rate of formation of the uniformly sized droplets.

Still other parameters include the temperature of the target or articlebeing formed and the state of the droplets as the droplets are depositedon the target or article being formed. The temperature of the dropletsand/or target and the drop velocity onto the target determine, in part,the bonding or merging of the droplet into a uniform layer on the targetmaterial.

In certain embodiments of the present invention, another parameter whichcan be varied is the "standoff" distance between the droplet formingmeans and the work station. This change in the distance affects both thesize of the "footprint" or impact area of the droplets and theliquid-to-solid fraction of the droplets. By varying the liquid-to-solidfraction, the bonding qualities of the droplets to the substrate can bechanged. It is to be understood that the temperature of the depositionmaterial itself affects the thermal state of the droplets. In certainembodiments, the temperature of the deposition material can vary fromjust above the melting point, and in other embodiments can be, forexample, about 50° C. above the melting point. This difference intemperature of the liquid deposition material will, of course, affectthe temperature of the droplets with respect to the distance from theorifice to the target (i.e., "in-flight" or "stand-off" distance).

The droplet is supplied at or about an optimum liquid-to-solid fraction.In various embodiments, the solid-liquid fraction of the droplet as thedroplet hits the target is an important variable. The impact of thedroplet on the target converts the kinetic energy of the descendingdroplet into heat energy. This heat remelts the droplet and the targetat the point of impact of the droplet. This remelting helps in thebonding process of the droplets to the newly formed surface.

In order to achieve geometric accuracy, the present invention preciselyaligns and focuses the droplet stream and carefully correlates the speedor direction of the target movement relative to the placement of thedroplet stream and to the flow rate and temperature of the dropletstream. The stream of droplets is controlled for efficiency andaccuracy. In a preferred embodiment, a suitable flow measuring system,such as a computer vision system operatively connected to a stroboscopiclight source, to estimate the flow rate of the metal by counting thenumber of droplets which pass a frame in a given time.

The positioning system moves the target according to the geometry of thesection or layer of the three-dimensional article or support beingcreated. The speed of motion of the target along a predefined path isgoverned by the flow rate of the deposition material, as preferablymeasured by the flow measuring system.

The number of layers and the positioning of the layers forming thethree-dimensional article is determined by a number of factors. Theobject geometry dictates certain points through which a layer must pass;thus, a minimum number of layers are needed in order to form thethree-dimensional article. It is to be understood that in variousembodiments where a support material is provided, the support materialis also being placed adjacent the partially fabricated article. Anotherfactor affecting the object geometry is the maximum thickness of eachlayer. It is to be understood that each layer can be no thicker thanthat portion of the article being formed. It is to be further understoodthat the functional requirements of the end use for thethree-dimensional article dictate the surface finish on thethree-dimensional article. Thus, the requirement for either asubstantially smooth surface finish or a textured finish can also limitthe layer thickness. It is to be understood, however, that the presentinvention provides a method for depositing droplets having a diameter assmall as approximately 50 microns such that the three-dimension articlesformed have a desirable surface for most end use applications. It is tobe further understood that, at the edges or periphery of the article,any staircase effect depends on not only the diameter of the droplet andtherefor the impending layer thickness, but also the spread or splashdiameter of the droplet as the droplet contacts the surface. In mostembodiments, the surface finish of the edges or periphery of the articlebeing formed have highly acceptable surface finishes having minimumstaircase effect which is suitable for most end use requirements.

It is to be understood that the actual size of the droplets beingdeposited depends upon the end use requirements of the three-dimensionalarticle. It is the accurate control of the deposition of the drops andthe size of each droplet which allows for the production ofthree-dimensional solid articles.

According to the method of the present invention, the deposition ofmaterial and the control over droplet size is accurate to withinfractions of a millimeter. This improvement in the positional accuracyand size of the droplets allows for the building of three-dimensionalarticles without recourse to intermediate physical models against whichto spray metal or to further processing steps. The three-dimensionalarticles formed according to the present invention are substantiallycontaminate-free and which can be directly utilized in industrial andcommercial applications.

The present invention forms droplets which are deposited in a singleoperation to form a three-dimensional article. The present inventioninvolves a net-shape process that rapidly produces an accurate, durablethree-dimensional article. Both the incremental application and thesubsequent solidification of the droplets occur in an accuratelycontrolled manner. The microstructure and the geometry of the articlebeing formed are precisely controlled so that no additional processingsteps such as machining are required in order to form thethree-dimensional article.

The droplets are supplied having a substantially uniform size, at apreferred velocity, and at a preferred distance from the target suchthat the droplet temperature and the target temperature are withinoptimally defined parameters. In particular, the droplets have apreferred diameter and are deposited at a preferred distance such thatthe liquid-to-solid fraction of the droplet is especially suited forbonding to the three-dimensional article. If the droplet is too cool,the droplet will form a powder-like material and will not bond to thearticle well. If the droplet is too liquid, then the liquid will flowand cause uneven and uncontrollable surfaces on the three-dimensionalarticle before cooling on the surface of the three-dimensional article.In a preferred embodiment, the liquid-to-solid fraction is approximately30:70 and the droplets have a substantially uniform size that varies indiameter no more than about ±25% and most preferably no more than about±5%.

In an especially preferred embodiment, the droplets are formed using anapparatus that forms uniform electrically charged molten metal droplets.The apparatus melts a charge of metal in a crucible and forces themolten metal through a small orifice (45-200 μm) in diameter to form alaminar jet or stream of droplets. The stream is broken by imposingvibrations (preferably from about 5 to 30 kHz) to a piezo-electrictransducer to form a stream of uniform droplets. Each droplet is chargedby a charging means such as a high voltage plate (about 300 to 400 V) aseach droplet breaks from the laminar jet or stream. The droplets aregiven a charge of the same polarity so that the droplets repel eachother so as to stay separate and thus maintain the original size.

In a preferred embodiment, the crucible is kept at a higher pressurethan the work space environment to force the liquid metal out throughthe orifice such that the pressure on the liquid controls the flow ofthe liquid through the small orifice.

One advantage of the present invention is that, it is now possible tocapitalize on the surface tension properties of the deposition metal tobuild overhang portions of material. The overhang portions compriselayers of droplets which extend beyond the edge of a previous layer toform an overhang structure.

Another advantage of the present invention is that the movement of thetarget work station can be around 3 to 5 axes. The target can be tiltedabout axes in addition to the linear movement along or between the X, Yand Z axes. This tilting allows the three-dimensional article to haveoverhang portions created without supports. For example, thethree-dimensional article can be rotated 90° to a position wherein thedeposition of the material continues to build vertically, but at a rightangle to the earlier deposited material, forming an overhang on the endproduct after the part is returned to its original positioning.

Another advantage of the present invention is that in certainembodiments, at least two materials can be used wherein one material isa sacrificial support material and the other material is the desiredobject material. The support material is dispensed adjacent the objectmaterial and is to be used only in supporting the overhang portions ofthe desired object material. The support material is removed upon of thecompletion of the three-dimensional object by applying heat, oxidizing,solvent, mechanical or other suitable means which are not harmful to thethree-dimensional article. The support material can comprise anysuitable material such as a lower melting point metal, an alloy, salt,glass, ceramic, graphite or a composite thereof.

Still another advantage of the present invention is that, in certainembodiments, when the distance from the droplets forming means to thetarget is sufficiently large, a supply of the droplets dispensed fromthat distance will solidify completely by the time the supply ofdroplets reaches the target. The supply of droplets which does solidifybefore reaching the target is, in essence, a supply of powder particleswhich acts as a support for subsequent layers of the depositionmaterial. The powder, by not having bonded to the article being formed,can be removed upon completion of the three-dimensional article.

Still another advantage of the present invention provides, in certainembodiments, a means for relieving stress in the layers of depositionmaterial. In certain preferred embodiments, a source of laser energy isused to anneal or relieve stress as the layers of material are dispensedto form the three-dimensional article and to control the immediate areasurface temperature.

The metal three-dimensional objects formed according to the presentinvention have desirable characteristics and properties which aresubstantially equivalent to or better than the properties of cast parts.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view, partially in cross-section and partiallyin phantom, of an apparatus for depositing molten material to form athree-dimensional article.

FIG. 1A is a greatly enlarged perspective view, partially incross-section of a portion of the apparatus shown in FIG. 1.

FIG. 1B is a greatly enlarged cross-sectional view of a droplet.

FIG. 1C is a perspective view of another embodiment of an apparatus fordepositing molten material to form a three-dimensional article.

FIG. 2 is a perspective view, partially in cross-section and partiallygreatly enlarged, of a portion of the apparatus as shown in FIG. 1showing one technique for depositing material and creating partoverhangs.

FIG. 3 is a perspective view, partially in cross-section, partiallygreatly enlarged and partially in phantom, of a portion of anotherembodiment of an apparatus for depositing molten metal to form athree-dimensional article and creating part overhangs.

FIG. 4 is a perspective view, partially in cross-section, partiallygreatly enlarged and partially in phantom, showing a portion of yetanother embodiment of an apparatus for depositing molten material toform a three-dimensional article and creating part overhangs.

FIG. 4A is a greatly enlarged, side elevational view, partially incross-section, of the article shown in FIG. 4 being formed.

FIG. 5 is a perspective view, partially in cross-section, partiallygreatly enlarged and partially in phantom, showing a portion of yetanother embodiment of an apparatus for depositing molten material toform a three-dimensional article and creating part overhangs.

FIG. 6 is a perspective view, partially in cross-section, showing aportion of another embodiment of an apparatus for depositing moltenmaterial to form a three-dimensional article.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, various embodiments of the process forforming a three-dimensional article and the apparatuses for use incarrying out the process will now be described in detail. As shown inFIG. 1, a three-dimensional object forming apparatus 10 is generallyshown. The apparatus 10 comprises at least one uniform droplet formationapparatus 12, and in the embodiment shown, comprises a further dropletformation apparatus 13. It is to be understood that the uniform dropletformation apparatus can be as described in the Chun et al. U.S. Pat. No.5,266,098, which reference is expressly incorporated herein. The uniformdroplet forming apparatuses 12 and 13 are enclosed in a chamber 15 in apreferred embodiment. The chamber 15 comprises a frame 8 having asuitable transparent shroud 9. The chamber 15 encloses an inertatmosphere and prevents undesired contaminants from coming into contactwith the molten metal as the three-dimensional article is being formed.However, it should be understood that in various embodiments other meansfor keeping contaminants away from the molten metal being deposited arewithin the contemplated scope of the present invention, and analternative embodiment showing a hemispherical chamber 15' is shown inFIG. 1C.

The uniform droplet forming apparatus 12 is, in one preferredembodiment, substantially similar to the uniform droplet formingapparatus 13. For ease of explanation, one set of numerals common toelements in each uniform droplet forming apparatus 12 and 13 will bedescribed.

The uniform droplet forming apparatus 12 and the apparatus 13 eachcomprise a vibrating means 16 and a crucible 18. It is to be understoodthat each crucible 18 has a heating means 19 to melt the depositionmaterial 14 to a desired temperature, and a moving means 21 to move thecrucible 18 in at least one, and in some embodiments, 3 directionsand/or between in the X, Y and Z axes. It is to be further understood incertain embodiments, that the molten material 14 within the crucible 18can be under a desired pressure from a pressurizing means 17. The moltenmaterial 14 in a preferred embodiment is subjected to vibrations by thevibrating means 16 at a desired amplitude and frequency. A stream or jet28 of material 14 is formed and exits the crucible 18 from at least oneorifice 20. The vibration of the stream 28 causes a plurality ofdroplets 36 having a substantially uniform size and shape to be formed.As the droplets 36 form, the droplets 36 pass through a charging system22. The charging system 22 generally comprises a charging plate 24having at least one opening 26 which is aligned with the orifice 20. Thecharging system 22 applies a charge to the droplets 36 as the droplets36 are being formed. As each droplet 36 breaks from the stream 28, eachdroplet 36 retains a portion of the charge.

As the droplets 36 descend, the droplets 36 pass through or adjacent afocusing or alignment means 30. In the embodiment shown, the alignmentmeans 30 can have a cylinder shape or conical shape (not shown) which isshown in cross-section for ease of illustration. The aligning means 30comprising a charging or repelling surface 32 which defines an opening34, as best seen in FIG. 1A. The charging or repelling surface 32 ispreferably made of a highly conductive material such as copper,aluminum, steel or the like and is, in certain embodiments, about 150 toabout 450 mm in length. The opening 34 is generally from about 10 toabout 40 mm in diameter. It is to be understood that in otherembodiments, the length and diameter of the repelling surface 32 aredependent, at least in part, on the type of material being deposited,the size of the droplets, and the ultimate shape of thethree-dimensional article.

When the charging or repelling surface 32 is held at a predetermineddesired voltage, the droplets 36 remain a predetermined distance fromeach other and from the charging or repelling surface 32. This repellingforce is generally shown by the double-headed arrows 35 in FIG. 1A. Aseach droplet 36 descends, a leading droplet 36A is repelled, not onlyfrom a succeeding droplet 36B, but is also repelled from the sides ofthe charging or repelling surface 32, thereby preventing thelike-charged droplets from merging with each other or scatteringsideways. The droplets tend to cluster around an axis extendinglongitudinally through the alignment means 30. The charge on thedroplets allows the droplets to be delivered in a fine, very accurateline.

It is to be understood that any suitable metal may be used dependingupon the end use application. The actual charge on each droplet is afunction not only of the type of metal used but also the diameter of thedroplet and the diameter of the opening 34 through which the chargeddroplets 36 descend and the voltage between the charging or repellingplate 32 and the droplets 36. A charge on the droplets 36 on the orderof 10⁻⁷ coulomb/gram is useful; however, it is to be understood thatother charges are also useful and that the charges depend on the variousparameters discussed above. The droplets at least partially solidifyduring the descent and are in a semi-liquid state at the point of impactat which the droplets reach a substrate or work station positioningsystem 40. As seen in FIG. 1B, as the droplet 36 cools a skin 37 isformed which shields a molten portion 39. In certain embodiments, as thedroplet 36 impacts either the work station positioning system 40 orpreviously deposited droplets, the skin breaks open and the dropletflattens.

In preferred embodiments, at the point of impact the droplet has anoptimal ratio of about 50:50 to about 20:80 and preferably 30:70 ofliquid-to-solid fraction of the droplet. The optimal ratio both ensuresthe accurate deposition of each individual droplet and prevents toogreat a remelting of the target or puddling of the liquid material atthe point of impact.

Referring again to FIG. 1, the work station positioning system 40comprises a plurality of moveable means for moving a work stationsurface 42. A first moveable means 44 moves the work station surface 42in a Y (forward and backward) direction as generally shown by the arrowY on the first moving means 44. A second moveable means 46 generallymoves the work station surface 42 in the X (horizontal or left andright) direction as generally shown by the arrow X shown on the movingmeans 46. A third moveable means 48 moves the work station in the Z(vertical or up and down) direction as generally shown by the arrow Z onthe means 48. In certain embodiments, a fourth moveable means 50generally further moves the work station positioning system 40 in the Xdirection between at least a first position or station and a secondposition or station. In the embodiment shown in FIG. 1, the firstdroplet forming apparatus 12 dispenses one type of molten material whilethe second droplet forming apparatus 13 dispenses a different type ofmolten material. The work station positioning system 40 is moveablebetween the droplet forming apparatuses 12 and 13 by the fourth moveablemeans 50. The work station positioning system 40 and the fourth moveablemeans 50 are operatively connected via a further moveable means 52 suchas a pneumatic or hydraulic means to a power source (not shown) formoving the work station positioning means 40. The work stationpositioning system 40 and the droplet forming apparatuses 12 and 13 areoperatively connected to a control means 56 having a planning system forproviding instructions for movement of the work station positioningsystem 40 and/or operating instructions for the droplet formingapparatuses 12 and 13.

The control means 54 can preferably have a computer software program orplanning system which reads a solid model representation of the objectgeometry and planning system sections this representation into a finitenumber of slices. The computer program coordinates the actuation of thework station positioning system 40 and monitors any sensor inputs suchas pressure, temperature, charge, feed rate, frequency, amplitude anddistances.

The method of the present invention results in the ability to produce,within a matter of hours, from scratch, a new and metal part havingoverhang portions directly from a CAD file. The three-dimensionalarticle has the strength and durability properties which are favorablycomparable to machined counterparts. Further, while the deposition asshown herein shows a single orifice 20, it is to be understood thatmultiple orifices can be utilized in the present invention depending onthe geometry of the three-dimensional article being formed.

It is further contemplated that the chamber 15 can comprise a vacuumchamber to withdraw any ambient air or oxygen from the depositionchamber. It is also contemplated that other devices having, for example,a much smaller glass dome or similar container, as shown in FIG. 1C, canbe used to evacuate ambient air, oxygen or the like from the depositionchamber 15 and place within the chamber an inert gas such as argon ornitrogen. The chamber can comprise a lower port 56 and an upper port 58to allow for the injection and evacuation of inert gas and/or ambientatmosphere.

In the embodiment shown in FIG. 1, the work station positioning system40 is accurately moveable in at least three planes such that, as thedroplets 36 are deposited in a predetermined pattern, each droplet 36builds on preceding deposited droplets to form a new surface 38three-dimensional article 60, as shown in FIG. 2.

The three-dimensional article 60 is generally shown as being formed of aplurality of flattened droplets 36C which form the new surface 38. Aseach droplet is deposited the droplets merge and form a vertical wall 62of the article 60. In this embodiment succeeding layers of droplets 36Dare formed such that the droplets overlap portions of the previouslydeposited droplets 36C. The succeeding droplets 36D impact on thepreceding droplets 36C such that the diameter of the droplets 36Doverlap the droplets 36C. In this manner, as succeeding rows of droplets36E, 36F, 36G and so on are formed, the droplets 36D, 36E, 36F and 36Gform an overhang portion generally shown as 64. As each droplet impactsthe previously deposited droplets and solidifies, the overhang portion64 is formed directly without need for any supporting substrate.

FIG. 3 shows an example of another embodiment wherein a support or workstation positioning system 140 having a planar surface or work station142 is rotatable about 5 axes such that the work station 142 can berotated in further dimensions. For example, the positioning system 140is shown rotated into a vertical (Y) direction such that the droplets 36can be deposited to form the complex three-dimensional shape of anarticle 70. It is to be understood that a first portion 72 of thearticle 70 can be formed while the planar surface 142 is in thesubstantially horizontal (X) plane. Thereafter, while the positioningmeans 140 is still in substantially the horizontal plane, a secondportion 74 is formed where the droplets are deposited to build thesecond portion 74 extending at substantially a right angle from thefirst portion 72. Thereafter, the positioning means 140 is rotated aboutan axis in an X-Y plane to allow a third portion 76 of the article 70 tobe formed at an angle. As is shown in FIG. 3, a fourth portion 78 of thearticle 70 is formed by rotating the positioning means 140 to a vertical(Y) direction such that the droplets are being deposited in a verticalmanner.

Referring now to FIG. 4, a further three-dimensional article 80 isgenerally being formed on a work station positioning system 240 having awork station surface 242. The positioning system 240 is preferablymoveable in at least three directions between the X, Y and Z axes. Inthe embodiment shown in FIG. 4, the first droplet forming means 12deposits droplets 82 of a first material 84 onto the work stationsurface 242 in a predetermined manner. In the embodiment shown, thesecond droplet forming means 13 contains a second or support material 90which deposits droplets 92 of the second material 90 onto or adjacentportions of the article 80 to act as a support material. As shown inFIG. 4A, the article 80 comprises a plurality of layers 80A which form aportion of the article 80. The second material 90 is deposited in areasadjacent the deposition material 80. The second material 90 then canreceive additional droplets 80B which are held in a position by thesecond material 90. Upon completion of the three-dimensional article 80the second material 90 can be removed by any means, as discussed above.

Referring next to FIG. 5, the droplet forming means 12 can be moved inthe X, Y and Z directions. The droplet forming means 12 is operativelyconnected to the moving means 21 and is moveable in at least a verticalor Z direction to raise the droplet forming means 12 such that thedistance between the droplets forming means 12 and a work stationpositioning system 340 holding a three-dimensional article 120 beingformed is increased. It is to be understood that the work stationpositioning system 340 can also be moveable in a Z or vertical direction(as shown in phantom in FIG. 5) to increase the distance between thedroplet forming means 12 and the article 120 being formed. The increasein distance between the droplet forming means 12 and the droplet impactarea allows a plurality of droplets 36Z to substantially solidify beforeimpacting on the target. As the droplets 36Z are solidified, thesolidified droplets 36Z form a powder particulate material 122 whichacts as a support material. Upon completion of the three-dimensionalarticle 120, the powder or support material 122 is removable from thethree-dimensional article 120. The distance and the rate at which thedroplets 36Z are deposited or metered is monitored such that thedroplets 36Z do not form or bond onto the article 120.

In various embodiments the powder support area 122 may not retain itsdesired configuration in order to act as a suitable support for the partoverhangs. In such situations, the powder support area 122 is preferablysupported by an exterior or interior wall 124 made from the moltenmaterial 36.

FIG. 6 further shows one embodiment of a means 300 for relieving stressin an article 280 being formed. It is to be understood that, while thestress relieving means 300 is shown in connection with a crucible system290 the stress relieving means 300 can be utilized in all embodiments inthe present invention and is being shown herein with one crucible systemfor ease of illustration. The stress relieving means 300, can comprise asource of laser energy. The stress relieving means 300 has variousdirecting means 302 and 304 for directing a beam 306 of laser light orenergy to a portion 286 of the material being deposited. The laser beam306 relieves stress in the material 280 being simultaneously depositedand bonded to preceding deposited layers to prevent any curling, warpingor other stresses from being built into the article 280. In addition,the laser beam 306 can be used to maintain a temperature control in theimmediate impact area as droplets 282 are deposited on thethree-dimensional article 280 being formed.

It is to be understood however, in certain embodiments, that othermethods to relieve stress in each layer such as shot peening, inductionheating or other annealing processes are also contemplated as beinguseful in the present invention.

It is also within the contemplated scope of the present invention thatin another embodiment the powder particles can be formed using twodroplet forming apparatuses; wherein one apparatus is positioned at afirst distance from the work station positioning system such that thedroplets form the three-dimensional article, and wherein the secondapparatus is positioned at a second greater distance from the workstation positioning system such that powder particles are formed.

According to the present invention, no further processing steps need becarried out on the finished article once the deposition process iscomplete. Each of the methods for forming a three-dimensional articledescribed herein can be used to produce a three-dimensional article ofany configuration, size and/or complexity.

While certain preferred embodiments have been shown and describedherein, it is to be understood that the invention is not limited theretobut may be variously embodied within the scope of the following claims.

We claim:
 1. A method for the accurate formation of a free-formthree-dimensional article without the use of a mold of thethree-dimensional article, the method comprising:providing a supply ofsubstantially uniform size droplets of a desired molten or semimoltenmetal material, each droplet having the same positive or negativecharge, aligning each droplet in the supply of droplets into asubstantially single file stream, the droplets being aligned by allowingthe droplets to pass through at least one alignment means, the alignmentmeans having the same charge as the droplets wherein the alignment meansrepels each droplet toward an axis extending through the alignmentmeans, depositing each of the aligned droplets in a pattern defining ashape of the three-dimensional article onto a target, the droplets beingdeposited onto the target along the axis extending longitudinallythrough the alignment means wherein the axis is substantiallyperpendicular to a top surface of the target, the deposited dropletsforming at least one layer of the three-dimensional article, wherein thethree-dimensional article comprises a plurality of the layers of thedeposited droplets, and reducing stress in the deposited droplets byannealing the deposited droplets as the deposited droplets form thenewly formed layer of the three-dimensional article, by using a sourceof laser energy to reduce the stress in the deposited droplets.
 2. Themethod of claim 1, in which each droplet is kept separate from adjacentdroplets by maintaining the positive or negative charge on each dropletuntil each droplet is deposited on the target or the newly formed layerof the three-dimensional article.
 3. The method of claim 1, in which thedroplets are passed through a substantially cylindrical orfrusto-conical shaped alignment means which substantially surrounds thenarrow stream of droplets.
 4. The method of claim 1, in which the targetis moveable in at least three directions.
 5. The method of claim 1, inwhich the supply of desired molten metal material is moveable in atleast one direction.
 6. The method of claim 1, in which the uniform sizedroplets are formed by vibrating a supply of the desired molten metalmaterial at a frequency and/or amplitude to accurately align eachdroplet in the supply of droplets into the narrow stream and toaccurately deposit the droplets in the pattern defining the shape of thethree-dimensional article.
 7. The method of claim 1, in which the targetis moveable relative to the supply of droplets to form thethree-dimensional article.
 8. The method of claim 1, in which thethree-dimensional article is formed from a computer model and in which,by a software program, the accurate deposition of the desired moltenmetal material is guided, wherein the software program accepts a solidmodel representation of the shape of the three-dimensional article,sections the representation of the three-dimensional article into afinite number of layers, and plots a deposition path needed to achieveeach layer.